The present invention relates to a method for the currentless catalytic precipitation of the aluminum of aprotic alan complex baths on substrate surfaces.
It is known that titanium tetrachloride causes considerable decomposition of aluminum-hydrogen compounds. According to the method described in the German Auslegeschrift No. 1,621,227, aluminum can be precipitated, in the presence of a decomposition catalyst, for example, one or several compounds of metals of the group IVB and VB of the periodic system of the elements, from an aluminum hydride compound such as complex alkali-, earth alkali- or magnesium-aluminum hydrides. It has been found, however, that such a catalyst does not adhere firmly on the substrate to be aluminized. Hence, the catalyst separates in the aluminizing bath from the substrate and results in the substrate being coated unevenly. Often the catalytic layer is too thick, and instead of the substrate being aluminized, the catalyst is separated in the aluminizing bath from the substrate and the immersion aluminizing bath is itself decomposed. Therefore, a technically usable aluminizing method cannot be realized in practice utilizing such a process since certain prerequisites regarding the activation of substrates and the aluminizing per se are lacking. Using the catalysts given in the German Auslegeschrift No. 1,621,227, the substrate to be aluminized cannot be coated with a firmly adhering catalytic film, a condition which has been found to be required for the precipitation of aluminum on the surface of the substrate. The materials named as catalysts in the German Auslegeschrift are not bonded to the substrate adsorptively, and, hence, they separate off in the aluminizing bath.